The inability to selectively prevent GVHD without diminishing graft-vs.-leukemia (GVL) effects limits the success of clinical allogeneic hematopoietic cell transplantation (allo-HCT), an effective therapy for the treatment of many otherwise fatal hematologic malignancies. CD8 T cells are major effector cells that induce both GVHD and GVL effects after allo-HCT. Our previous studies using anti-IFN-gamma mAb and IFN-gamma-deficient mice showed that IFN-gamma restricts donor CD8 T cell activation/expansion through induction of apoptosis and inhibits mononuclear cell infiltration in parenchymal GVHD target tissues in recipients of CD4-depleted allo-HCT, and that donor CD8 T cells induce more severe GVHD but less potent anti-lymphohematopoietic GVH reactions and GVL effects in the absence of IFN-gamma. The goal of this project is to understand the mechanisms by which IFN-gamma regulates the alloreactivity of donor CDS T cells as a prerequisite to the development of approaches that can induce GVL effects without severe GVHD. In Aim 1, we will determine whether or not IFN-gamma restricts expansion of both recipient antigen-specific and -nonspecific (homeostatic proliferation) donor CDS T cells, and identify the mechanisms responsible for IFN-gamma-induced apoptosis of donor CDS T cells in allo-HCT recipients. In Aim 2, we will test the hypothesis that IFN-gamma suppresses the migration, in-situ proliferation and/or survival of donor CDS T cells in parenchymal GVHD target tissues and thereby inhibits GVHD while preserving GVL effects. In Aim 3, we will determine how IFN-gamma mediates GVL effects without promoting GVHD. We will: 1) develop IFN-gamma-unresponsive leukemia models to determine whether or not IFN-gamma can enhance GVL effects by inhibiting tumor cell proliferation and/or sensitizing tumor cells to alloreactive CTLs; 2) assess the role of IFN-gamma in the differentiation of alloreactive CTLs with different cytotoxic mechanisms and the contribution of these cytotoxic mechanisms to the induction of GVHD vs. GVL effects in the presence and absence of IFN-gamma; and 3) determine the role of NK and NKT cells in the induction of GVL effects. Achieving these aims will lead to a better understanding of mechanisms involved in GVHDand GVL-associated alloreactivity of CDS T cells and facilitate the development of novel protocols for the performance of HLA-mismatched allo-HCT in the clinic. In addition to highly-relevant hypotheses to be addressed, collaborative interactions within the PPG can also be established based on the models developed in this project.